Imperative type definition

### 🔍 Search Terms

imperative typescript interface

### ✅ Viability Checklist

- [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [x] This wouldn't change the runtime behavior of existing JavaScript code
- [x] This could be implemented without emitting different JS based on the types of the expressions
- [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

I work on a library that requires to use lots of 3rd party generic types, multiple mappings and more or less complex typing, and I've got to repeat one complex type multiple times or create another generic that is going to contain a lot of parameters and will complicate the code that's already complicated.

(these types have bugs, they just an illustration)

### 📃 Motivating Example

I imagine it as an "type function" that has function body that returns any valid type.

```ts
imperative type Foo {
type X = Bar;
type Y = Baz;

return { x: X, y: Y };
}

type Quix = Foo;
// Quix { x: Bar, y: Baz }

// ... vs
interface Foo {
x: Bar;
y: Baz;
}
```

### 💻 Use Cases

- Imperative style
- Shorter syntax for complex types

No implementation file, entry point, or test is identified. Start by reviewing the proposed syntax and motivating examples against TypeScript's design goals, then inspect the type-checker architecture to determine where such a type-level feature would fit; done requires an agreed design and corresponding implementation and tests.

Written by the indexing model from the issue text.